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Abstract:

Expected stringent legislation on particulate matter (PM) emission by gas turbine combustors is currently motivating considerable efforts to be better understand, model and predict soot formation. This complex phenomenon is very difficult to study in detail with experiment, and numerical simulation is an essential complementary tool. Considering that the chemistry of soot particles strongly depends on their size, the numerical prediction of soot formation requires the description of their size distribution. To do so, either Eulerian methods (sectional or moments) or stochastic Lagrangian approaches are reported in the literature. In the present work, a far more simple semi-deterministic Lagrangian approach is proposed. An accurate description of the gaseous phase including first Polycyclic Aromatic Hydrocarbons is also developed as a necessary input to detail soot model. This work aims to develop a viable methodology of soot description within the LES framework. The manuscript is organized into three parts. The first part introduces the context and presents a literature review of soot particles focusing on numerical soot modelling. Among the existing method, the Lagrangian soot tracking is retained where additional developments are required to describe the particle size distribution (PSD). Then, the second part deals with laminar sooting flames. The modelling of reactive flow is briefly described, and the choice of chemistry modelling is also discussed in details. The Analytically Reduced Chemistry (ARC) is retained for the chemical description. Several ARC including PAH chemistry are selected, derived and validated on canonical laminar flames for different fuels, targeting different PAH. Lagrangian soot tracking has been developed and validated on canonic flames compared to a well-established method from the literature for which excellent agreement is found. The combination of ARC chemistries with Lagrangian soot tracking has been applied to investigate a set of canonic laminar flames analyzing soot global quantities and PSD. Good predictions are obtained with the proposed methodology. Finally, the last part presents the soot prediction obtained with the proposed methodology in two complex configurations representative of an aeronautical combustors. The first one is the FIRST configuration, a gaseous confined pressurized swirled flame studied experimentally at DLR. Impact of precursors species and radiative transfers through the resolution of Radiative Transfer Equation (RTE). Good predictions are obtained compared to experiments for predicted temperature and soot volume fraction. The second target configuration is the UTIAS Jet A-1 burner and corresponds to a confined turbulent spray flame burning aviation jet fuel A-1 studied experimentally at UTIAS Toronto. LES of this configuration provides a qualitative and quantitative understanding of soot evolution in turbulent spray flames. Numerical predicted soot volume fraction using Lagrangian soot tracking and an ARC mechanism including pyrolysis method is compared to experimental measurements. Results show the ability of the proposed methodology relying on ARC chemistry for Jet A-1 including pyrolysis method and Lagrangian soot tracking, to predict accurately soot compared to available measurements. In addition to an accurate soot model, the present work highlights the requirement of an accurate chemical description especially concerning soot inception as well as an accurate description of heat transfers for future investigation in turbulent flames.

Keywords: Soot, combustion, LES, Radiative transfer, Lagrangian formalism, Gas turbines

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